The insulating material of a stator coil of a rotator is obtained by winding a mica tape made of mica, a reinforcing member and a small amount of resin component for bonding the mica and the reinforcing member together around a coil conductor formed by binding a plurality of insulation-coated electric wires, and impregnating the resultant with a thermosetting resin composition, which is then hardened by heating.
In recent years, there have been increasing requests for a rotator to be reduced in size, increased in performance and improved in reliability, which causes concerns about dielectric breakdown and the like during operation. Accordingly, the insulating materials for such rotators are required to have reliability so as to be excellent in both of the physical stability and the chemical stability in a relatively higher temperature region of 200° C. or higher. In particular, there is a strong demand for an insulating material of a rotator coil for a vehicle that fulfils a required insulation function even under severe temperature conditions.
One conceivable method of obtaining a coil insulation material having high heat resistance is a method of blending a chemically highly resistant composition into a thermosetting resin composition used for impregnation. For example, there may be a method of adding a maleimide compound, an isocyanuric acid compound or the like that is a high heat-resistant compound to a bisphenol A-type epoxy resin that is a general-purpose impregnating resin material (PTD 1: Japanese Patent Laying-Open No. 2009-112159).
However, in the state where only the heat resistance of the thermosetting resin composition is improved, when a rotator is used at an elevated temperature of 200° C. or higher, dielectric breakdown may occur in a coil insulation material portion within 20,000 hours that is an indicator of a heat-resistant life.
As a result of reviewing several factors of dielectric breakdown described above, we have found that an impregnating resin (a hardened material of the thermosetting resin composition) is thermally decomposed and generates gas while deteriorating at a relatively high temperature, thereby decreasing the adhesiveness between the mica and the impregnating resin and causing peeling-off at this portion, with the result that dielectric breakdown occurs. Accordingly, in order to ensure the long-term reliability of coil insulation, it is necessary to increase the heat resistance of the impregnating resin and also improve the adhesiveness between the mica and the impregnating resin.
In PTD 1, the heat resistance of the coil insulation material is improved by using a thermosetting resin composition obtained by blending high heat-resistant bismaleimide and the like into an epoxy resin. However, since bismaleimide is lower in adhesiveness to mica than an epoxy resin, mica paper 7 and a reinforcing member 6 are peeled-off from each other during the operation of the rotator. Thus, the heat resistance of the coil insulation material cannot be sufficiently improved (see FIG. 3).
Furthermore, the adhesiveness between the mica and the impregnating resin can be improved by blending an acrylic resin and the like excellent in adhesiveness into a thermosetting resin composition. In this case, however, it is necessary to increase the concentration of the added acrylic resin and the like, which may cause an adverse effect on the heat resistance and the electrical property.